The Kidde 900-0113 AC powered, plug-in CO and explosive gas alarm protects you and your family from two deadly threats. The 900-0113 includes 9V battery back up that provides protection during a power outage, when AC-only units can not provide protection. By pressing the Peak Level Button you can see the peak CO level recorded by the alarm since it was last reset or unplugged. The continuous digital display shows you the level of carbon monoxide (if any) the unit is sensing as well as if gas is present. The gas sensor is a metal oxide sensor designed to detect natural gas (methane) or propane.